Water of crystallisation

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Presentation transcript:

Water of crystallisation

Water of crystallisation 1. Explain the terms anhydrous, hydrated and water of crystallisation 2. Calculate the formula of a hydrated salt using percentage composition, mass composition or experimental data. Keywords hydrated anhydrous dot formula

Hydrated copper (II) sulphate Anhydrous copper (II) sulphate Hydrated copper (II) sulphate CuSO4 . 5H2O CuSO4 ______________ ______________

Water molecules are trapped within the crystal structure This is called water of crystallisation CoCl2.6H2O

CuSO4 5H2O CoCl2 6H2O Na2SO4 10H2O copper sulphate pentahydrate cobalt chloride hexahydrate Na2SO4 10H2O sodium sulphate decahydrate

What happens if we don’t know the dot formula? Rule: use the hydrogens to work the dot formula out NOT the oxygens Salt Empirical Formula Dot Formula Hydrated magnesium chloride MgCl2H10O5 Hydrated sodium carbonate Na2CH20O13 Hydrated calcium nitrate CaN2H8O10 MgCl2 5H2O Na2CO3 10H2O Ca(NO3)2 4H2O

What happens if you’re told nothing but still have to work out the water of crystallisation?! Find the n in BaCl2 . nH2O

You have to do it experimentally: Use heat to drive off the water! This makes the BaCl2 anhydrous. Mass of crucible: 30.00 g Mass of crucible + BaCl2.nH2O crystals: 32.44 g Mass of crucible + anhydrous BaCl2: 32.08 g Mass of BaCl2 : 2.08 g Mass of H2O : 0.36 g

You have to do it experimentally: Use heat to drive off the water! This makes the BaCl2 anhydrous. Mass of crucible: 30.00 g Mass of crucible + BaCl2.nH2O crystals: 32.44 g Mass of crucible + anhydrous BaCl2: 32.08 g Mass of BaCl2 : 2.08 g Mass of H2O : 0.36 g

3. Number of moles (mass / M) 4. Ratio 5. Empirical formula (RAMs: H=1, O=16, Cl = 35.5, Ba = 137 ) Substances Anhydrous BaCl2 H2O 1. Masses (g) 3. Number of moles (mass / M) 4. Ratio 5. Empirical formula 2.08 0.36 2.08/208 = 0.01 0.36/18 = 0.02 Divide by smallest 1:2 BaCl2 . 2 H2O

Water of Crystallisation Questions: You are supplied with three empirical formulae. Write down the dot formula of each salt to show the water of crystallisation. a) BaCl2H4O2 b)ZnSH14O11 c) FeN3H12O15 2) From the data below work out the formula of the hydrated salt. Mass of CaCl2.xH2O = 6.573 g Mass of CaCl2 = 3.333 g

CaCl2.nH2O: 6.573 g anhydrous CaCl2: 3.333 g Mass of H2O : 3.24 g (RAMs: H=1, O=16, 40 = 35.5, Cl = 35.5 ) Substances Anhydrous CaCl2 H2O 1. Masses (g) 3. Number of moles (mass / M) 4. Ratio 5. Empirical formula 3.333 3.24 3.333/111 = 0.03 3.24/18 = 0.18 Divide by smallest 1:6 CaCl2 . 6 H2O

Example questions A sample of hydrated magnesium sulfate was strongly heated. The mass of magnesium sulfate before and after heating were recorded in the table below. Calculate the formula of hydrated magnesium sulfate Mass of samples before heating (g) 1.232 Mass of sample after heating (g) 0.602 Mass of water lost during heating (g)

Example questions A sample of hydrated zinc sulfate was strongly heated. The mass of zinc sulfate before and after heating were recorded in the table below. Calculate the formula of hydrated zinc sulfate Mass of samples before heating (g) 2.875 Mass of sample after heating (g) 1.615 Mass of water lost during heating (g)

Example questions A sample of hydrated magnesium chloride was strongly heated. The mass of magnesium chloride before and after heating were recorded in the table below. Calculate the formula of hydrated magnesium chloride Mass of samples before heating (g) 3.706 Mass of sample after heating (g) 1.906 Mass of water lost during heating (g)

Plenary Complete the exam style questions on water of crystallisation, ionic and covalent bonding